1. Field of the Invention
The invention relates to a device with a heat exchanger with a feed pipe for a medium leading from a medium inlet to the heat exchanger entrance and with a discharge pipe leading away from the heat exchanger exit.
2. The Prior Art
Such type heat exchangers are needed in many applications. The transferred energy is thereby determined by the different temperatures of the media that are carried in the heat exchanger. Different control mechanisms are known for varying the volume flow of these media. Since it is frequently necessary to achieve certain medium temperatures without it being possible, as a rule, to modify the surface of the heat exchanger, the flow speed in the heat exchanger is varied.
An alternative to this can be to operate the heat exchanger in a concurrent or a countercurrent flow. While the medium temperatures at the heat exchanger exit can converge strongly in the concurrent flow operating mode, the countercurrent flow operating mode provides, as a rule, a higher heat exchange with the same heat exchanger surface. Using the switch from concurrent flow to countercurrent flow as a control mechanism must be rejected, since the piping is already determined during installation of the heat exchanger and cannot be changed during operation.
A specific field of application of particularly big heat exchangers is the heating and cooling of gases of firing systems which are used as steam generating plants. In such plants, the air supplied to the fire grate, respectively to the combustion area, must be preheated and the exhaust gases are cooled. Heat exchangers are thereby used as evaporators and superheaters, in order to supply a turbine with steam. The feed water of the steam generator is frequently preheated in an economizer to further cool the exhaust gases.
During the operating time of the steam generating plant, the exhaust gas temperature varies in accordance with the combustion process. Furthermore, deposits occur in the evaporator and in the superheaters, thus compromising the effectiveness of the heat exchangers. The economizer is thereby eventually exposed to different exhaust gas temperatures. The effectiveness of the economizer furthermore also varies according to the deposits produced by the exhaust gases in the pipes of the heat exchanger.
Most of the time, a denitrogenization plant, the catalytic effects of which only take place in an optimal manner at certain temperatures, is provided downstream of the economizer. In SCR plants for instance, these temperatures lie between 250° C. and 270° C.
During the first operating hours of such a plant, the heat exchangers still have a high effectiveness which however decreases during the operating time due to deposits. The run time of the plant is more specifically also determined by the fact that the exhaust gas temperature at the denitrogenization plant must remain inside a determined temperature window.